1. Technical Field
The present invention relates to a semiconductor device, a method of cutting an electrical fuse, and a method of determining an electrical fuse state. More specifically, the present invention relates to a semiconductor device including an electrical fuse, a method of cutting an electrical fuse, and a method of determining an electrical fuse state.
2. Related Art
A technology is conventionally known, in which a fuse is mounted in a semiconductor device, and the fuse is broken, so that a value of a resistance employed in the semiconductor device is suitably adjusted, or a failure element is separated and replaced into a normal element.
Typical manner for cutting a fuse includes cutting a fuse by irradiating a laser beam in a portion of the fuse and cutting a fuse by applying an electric current.
Japanese Laid-Open Patent Publication No. 2005-57186 discloses an electrical fuse that is cut by utilizing the phenomenon that constituent materials of an electrical fuse move by electromigration. Furthermore, the Japanese Laid-Open Patent Publication No. 2005-57186 discloses a configuration of the electrical fuse in which a cutting target region is surrounded by a plate, thereby confining or accumulating heat generated in the cutting target region when current is applied to the electrical fuse and accelerating cutting the electrical fuse.
Japanese Laid-Open Patent Publication No. 2004-103610 discloses a trimming pattern (an electrical fuse) for making an adjustment while selecting on or off for the connection of an adjustment circuit prepared in a semiconductor integrated circuit in advance. FIG. 10 is a schematic plan view of the trimming pattern disclosed in the Japanese Laid-Open Patent Publication No. 2004-103610. The trimming pattern includes two pads 11 and 12 to which a voltage is applied, a fine interconnect 10 connecting the two pads 11 and 12, and two connecting portions 13 and 14 which are located on each of both sides of the fine interconnect 10 without making contact with the fine interconnect 10 and connected to the circuit for adjustment and the semiconductor integrated circuit. Trimming is carried out using this trimming pattern not by applying a voltage between the pads 11 and 12 to flow a current so that the fine interconnect 10 is melted and cut and thereby turning off the circuit for adjustment, which is connected to the pads but in accordance with a method for connecting the melted metal of the fine interconnect 10 to the connecting portions 13 and 14, and thus, turning on the circuit for adjustment, which is connected to the connecting portions 13 and 14. It is easier to make the melted metal contact with the connecting portions 13 and 14, which are close by, than to melt and cut the fine interconnect 10, and thus, it can be said that trimming can be carried out easier and in a short period of time.
The present inventor has recognized that the conventional techniques stated above have the following problems.
If the electrical fuse configured as disclosed in the Japanese Laid-Open Patent Publication No. 2005-57186 is cut, then a resistance of a cutting region is increased, and a current value of the current flowing through the electric fuse is reduced. With the conventional techniques, it is determined whether the electrical fuse has been cut by determining whether a potential of one terminal of the electrical fuse is kept after a predetermined potential is applied to the terminal. FIG. 11 shows configurations of a conventional electrical fuse and a conventional determination circuit.
An electrical fuse 1001a includes a conductor 1002a and terminals 1004a and 1006a arranged on both ends of the conductor 1002a. The terminal 1006a of the electrical fuse 1001a is connected to a determination circuit 1010a. Further, the terminal 1004a of the electrical fuse 1001a is connected to a common interconnect 1020 common to and a terminal 1004b of another electrical fuse 1001b. The electrical fuse 1001b is similar in configuration to the electrical fuse 1001a. 
Next, a method of determining whether an electrical fuse has been cut will be described. It is assumed that the electrical fuse 1001a is in a sufficiently cutting state and that the electrical fuse 1001b is in an insufficiently cutting state. If it is to be determined whether the electrical fuse 1001a has been cut, then the common interconnect 1020 is grounded, and a predetermined potential is applied from the determination circuit 1010a to the terminal 1006a, thereby detecting a potential of the terminal 1006a of the electrical fuse 1001a. If the electrical fuse 1001a is not cut, then a current is flown between the terminals 1006a and 1004a and the potential of the terminal 1006a is not kept because the common interconnect 1020 is grounded. On the other hand, if the electrical fuse 1001a is cut, no current is flown between the terminals 1006a and 1004a. Therefore, the potential of the terminal 1006a is kept and the determination circuit 1010a detects charge. As a result, it is correctly determined that the electrical fuse 1001a has been cut.
However, if for example, the electrical fuse 1001b is incompletely cut although a cutting processing has been performed on the electrical fuse 1001b, the following problems occur. A current is flown between the terminals 1006b and 1004b, the potential of the terminal 1006b is not kept, and it is erroneously determined that the electrical fuse 1004b is not cut.
To avoid such an erroneous determination, a higher potential may be applied from the determination circuit to the terminal. If the higher potential is applied, the following new problems occur. Generally, a plurality of electrical fuses included in a semiconductor device is connected to one another by a common interconnect for area saving. FIG. 12 shows a configuration in which three electrical fuses (electrical fuses 1001a, 1001b, and 1001c) are connected to one another by a common interconnect 1020. If one of the electrical fuses 1001a, 1001b, and 1001c is to be cut, then a power supply voltage is applied to the common interconnect 1020, the electrical fuse to be cut is selected, an end of the common interconnect 1020 opposite to an end to which the power supply voltage is applied is grounded, and current is flown through the electrical fuse. If it is to be determined whether the electrical fuse has been cut, then the common interconnect 1020 is grounded, and a predetermined potential is applied to terminals from determination circuits. Furthermore, current flowing through each of the electrical fuses is detected to thereby determine whether one of the electrical fuses has been cut. At this time, the current flows into the common interconnect 1020 from the uncut electrical fuses. If the potentials applied to the terminals from the corresponding determination circuits are higher, an amount of the current flowing from many electrical fuses increases. As a result, the potential of the common interconnect 1020 floats. As a result, it is erroneously determined that the electrical fuses have been cut although they are not actually cut.
According to the technique disclosed in the Japanese Laid-Open Patent Publication No. 2004-103610, it is determined whether the electrical fuse has been cut by contacting the molten metal with the connecting portions 13 and 14 near the fine interconnect 10 instead of melting and cutting off the fine interconnect 10 of the trimming pattern. However, if the fine interconnect 10 is not cut when current is flown through the trimming pattern, the following problems occur.
FIGS. 13A and 13B show a configuration in which a plurality of electrical fuses (A, B, and C) similar to the electrical fuse (trimming pattern) disclosed in the Japanese Laid-Open Patent Publication No. 2004-103610 is arranged. Generally, a plurality of electrical fuses is arranged in parallel on a semiconductor substrate. Further, the electrical fuses are connected to a common interconnect for suppressing an increase in area to which the electrical fuses are arranged. Here, an instance in which pads 11 are connected to one another by the common interconnect is shown. If a voltage is applied to melt a fine interconnect 10 of an electrical fuse, then a voltage is applied to the common interconnect, the pad 12 of the electrical fuse is selectively grounded, for example, and current is flown through the fine interconnect 10. The fine interconnect 10 is thereby molten.
For example, in each of the electrical fuses A and C, a molten metal 20 of the fine interconnect 10 is in contact with connecting portions 13 and 14 (see FIG. 13A). With such a configuration, to determine whether the electrical fuse A has been cut, the connecting portion 13 is grounded and a potential is applied to the connecting portion 14. If the fine interconnect 10 is not cut from the pad 11 in the electrical fuse A, then a current is flown from the pad 11 of the electrical fuse A via the common interconnect, and the voltage is transmitted to the connecting portion 14 of the electrical fuse C via the pad 11 of the electrical fuse C (see FIG. 13B). If such an unintended transmission path is occurred, the potential applied to the electrical fuses from corresponding determination circuits causes a potential of the common interconnect to float. As a result, it cannot be correctly determined whether the electrical fuse A has been cut.
Moreover, with the configuration disclosed in the Japanese Laid-Open Patent Publication No. 2004-103610, it is necessary to provide the two connecting portions 13 and 14. As a result, an area necessary for formation of the trimming pattern disadvantageously increases.